Computer programs for generating 1-bit image data comprising “on” and “off” pixel values from multiple-bit image data, which 1-bit image data, when printed, produce an image that is constituted by dots, are well known, the process of generating the 1-bit image data being known as “screening”.
Such computer programs fall into two broad categories. The first category carry out so-called “amplitude modulated” (AM) screening to generate 1-bit image data that, when printed, produce an image that is constituted by a plurality of sizes of dots, the dots being regularly spaced throughout the image.
The second category carry out so-called “frequency modulated” (FM) screening to generate 1-bit image data that, when printed, produce an image that is constituted by a plurality of densities of dots, the dots typically all having the same size.
There are various problems with AM screening, the most significant of which are colour shift and formation of Moiré patterns when two or more screened images are overlaid, as occurs in colour printing.
These problems are caused by the regular spacing of the dots and do not arise in FM screening. There are, however, different problems with FM screening, the most significant of which are the appearance of graininess of the image and the appearance of patterning of the image.
The appearance of graininess occurs because of the irregular spacing of the dots throughout the image. Whereas the human eye readily disregards the regularly spaced individual dots making up an image produced by AM screening, it struggles to disregard the irregularly spaced individual dots making up an image produced by FM screening, which gives the image the appearance of graininess. This is exacerbated where the image contains clusters of adjacent dots among irregularly spaced individual dots, as such clusters are extremely conspicuous.
Each pixel value of the 1-bit image data corresponds to a pixel of an output medium. When printing the 1-bit image data, an output device attempts to mark those pixels of the output medium which correspond to “on” pixel values of the 1-bit image data. Although a few output devices can fully mark a pixel of the output medium without marking adjacent pixels, most output devices either do not fully mark the pixel, which gives rise to so-called “dot loss”, or also mark adjacent pixels, which gives rise to so-called “dot gain”.
The appearance of patterning results from dot gain because pairs of diagonally adjacent marked pixels exhibit more dot gain than pairs of horizontally or vertically adjacent marked pixels. Dot gain also causes isolated pixels corresponding to “off” pixel values of the 1-bit image data, referred to in this specification as “non-pixels”, i.e. non-pixels surrounded by marked pixels, to have a greater proportion of their areas marked, if not all of their areas, as a result of marking the surrounding pixels, than the non-pixels of isolated pairs of horizontally or vertically adjacent non-pixels.
FIG. 1 represents a portion 10 of an output medium on which a pair of horizontally adjacent pixels 12 and 14, a pair of vertically adjacent pixels 16 and 18, and a pair of diagonally adjacent pixels 20 and 22 have been marked by an output device that produces dot gain. As can be seen from FIG. 1, marking of the pair of horizontally adjacent pixels 12 and 14 causes six regions 24, 26, 28, 30, 32 and 34 of the surrounding pixels also to be marked as a result of dot gain. Marking of the pair of vertically adjacent pixels 16 and 18 also causes six regions 36, 38, 40, 42, 44 and 46 also to be marked. Marking of the pair of diagonally adjacent pixels 20 and 22, however, causes eight regions 48, 50, 52, 54, 56, 58, 60 and 62 also to be marked. The pair of diagonally adjacent marked pixels 20 and 22 would therefore appear darker than the pairs of horizontally and vertically adjacent marked pixels 12 and 14 and 16 and 18.
Portions of an image containing isolated pairs of diagonally adjacent marked pixels, such as marked pixels 20 and 22 of FIG. 1, or multiple lines of diagonally adjacent marked pixels, can have an appearance of patterning, because the isolated pairs of diagonally adjacent marked pixels and the multiple lines of diagonally adjacent marked pixels appear darker than nearby pairs of horizontally and vertically adjacent marked pixels. Portions of an image containing both isolated non-pixels and isolated pairs of horizontally and vertically adjacent non-pixels can also have an appearance of patterning, because the isolated pairs of horizontally and vertically adjacent non-pixels appear much lighter than the isolated non-pixels.
For these reasons of graininess and patterning, although FM screening has the potential to produce better printed images than AM screening, it has not been as widely accepted as AM screening.